To charge an energy storage capacitor, a d.c. voltage source is required to deliver an adequate output voltage. The voltage of the energy storage capacitor at the end of the charging cycle must be adjusted as accurately as possible to allow for accurate exposure control. Also, charging must be completed in a relatively short time, but the charging current must be prevented from becoming excessive at the start of the charging cycle or the components used will become unserviceable.
U.S. Pat. No. 4,325,008 disclosed a charging circuit in which an a.c. voltage is used as the input voltage. This a.c. voltage is stepped up by means of a transformer and then rectified by a bridge rectifier. A relatively complicated transformer is used with current limiting elements (shunts). The transformer also has an auxiliary winding which is short-circuited by a TRIAC activated as soon as the capacitor has reached the required voltage by a signal from a control circuit. This circuitry ensures rapid disconnection of the charging current when the required voltage has been reached.
If a d.c. voltage is available as the output voltage, the charging current souce is usually constructed in the form of a d.c. converter, such as a push-pull converter using a transformer whose primary windings have the input current flowing through them in phase opposition by means of a suitable circuit. Under these conditions, the converter is required to satisfy high current loading requirements and the circuit must have high switching accuracy to avoid any destruction of the converter by short-circuits due to inaccuracies in the closing and opening of the switches in the primary circuit.
U.S. Pat. No. 4,307,440 discloses a controlled d.c. source with a two-stage construction in which the output voltage of the first stage is regulated and then stepped up by a series-connected d.c. converter. The output voltage of the first stage is taken from a capacitor charged by means of a pulsed charging current. Voltage regulation is obtained by varying the duty cycle of the charging current. This voltage source, however, requires numerous protective devices to limit the switching current.
Other regulated voltage supplies are disclosed in European Patent Application No. 0176425 and European Patent Application No. 0112763, while U.S. Pat. No. 4,099,225 describes a protective circuit for an inverter. French Patent Application No. 2.416.617 describes a converter circuit for the voltage supply of discharge lamps so that the lamps' intensity is independent of the supplied voltage.
It would be desirable to provide a charging circuit having great operational reliability while using simple circuitry which allows for the accurate regulation over a wide range of the charging current used to charge energy storage capacitors.